Method for fabricating a heat pipe, and instrument of the method

ABSTRACT

The disclosure provides a method for fabricating a heat pipe, and an instrument of the method. The method for fabricating a heat pipe includes providing a hollow tube, wherein the hollow tube has an open end and a closed end; disposing a mandril into the hollow tube from the open end, wherein the inside wall of the hollow tube is separated from the mandril by a space, and wherein the mandril comprises a first portion and a second portion and the first portion has a thermal expansion coefficient larger than that of the second portion; filling up the space between the mandril and the hollow tube with a powder; performing a sintering process to the hollow tube, forming a first agglomeration region and a second agglomeration region; removing the mandril; injecting a working fluid into the hollow tube; and sealing the open end of the hollow tube.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Taiwan Patent Application No. 100129692, filed on Aug. 19,2011, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Technical Field

The disclosure relates to a method for fabricating a heat pipe and aninstrument of the method, and in particular relates to a method forfabricating a heat pipe with high heat transfer performance and aninstrument of the method.

2. Related Art

Heat pipes have excellent heat transfer performance due to their lowthermal resistance, and are therefore an effective means for thetransfer or dissipation of heat from heat sources. Currently, heat pipesare widely used for removing heat from heat-generating components suchas central processing units (CPUs) of computers.

Generally, according to the positions from which heat is input oroutput, the heat pipe has three sections: an evaporating section, acondensing section and an adiabatic section between the evaporatingsection and the condensing section.

The adiabatic section is typically used for transport of the generatedvapor from the evaporating section to the condensing section. When theevaporating section of a heat pipe is thermally attached to aheat-generating electronic component, the working fluid receives heatfrom the electronic component and evaporates. The generated vapor thenmoves towards the condensing section of the heat pipe under the vaporpressure gradient between the two sections. In the condensing section,the vapor is condensed to a liquid state by releasing its latent heatto, for example, a heat sink attached to the condensing section. Thus,the heat is removed away from the electronic component.

Then the condensed liquid (the working fluid has high specific heatcapacity, density, and low viscosity) flows to the evaporating sectionalong the capillary configuration of the heat pipe. Thisevaporating/condensing cycle repeats and since the heat pipe transfersheat so efficiently, the evaporating section is kept at or near the sametemperature as the condensing section of the heat pipe. Correspondingly,the heat-transfer capability of the heat dissipation device includingthe heat pipe is improved greatly.

The heat transfer performance of the heat pipe depends on three majorparameters: pore size between powders for forming the heat pipe,porosity of the heat pipe, and permeability of the heat pipe. Regardinga sintered heat pipe, the above parameters can be adjusted by followingprocess conditions: geometry of the sintering powder, average pore sizeof the sintering powder, and the sintering period.

The evaporating section, condensing section and adiabatic section of aheat pipe respectively have specific requirements of the three majorparameters. Ideally, the evaporating section of the heat pipe shouldhave a lower particular size of sintering powder, and the condensingsection and adiabatic section of the heat pipe should have a higherparticular size of the sintering powder, since the evaporating sectiondemands high capillary force for recovering the working fluid and broadevaporation area for performing a heat exchange, and the condensingsection and adiabatic section demand low fluid impedance forfacilitating the liquid transport.

In order to obtain a heat pipe having portions with different particularsizes of sintering powders, a conventional method employs at least twopowders with different particular sizes for fabricating a heat pipe. Theabove method, however, leads to a low product yield and increases theprocess complexity, and is not suitable for mass production.

SUMMARY

An exemplary embodiment of a method for fabricating a heat pipe includesthe following steps: providing a hollow tube, wherein the hollow tubehas an open end and a closed end; disposing a mandril into the hollowtube from the open end, wherein the inside wall of the hollow tube isseparated from the mandril by a space, and wherein the mandril comprisesat least one first portion and at least one second portion and the firstportion has a thermal expansion coefficient larger than that of thesecond portion; filling up the space between the mandril and the hollowtube with a powder; performing a sintering process to the hollow tube,forcing the powder to form at least one first agglomeration region andat least one second agglomeration region, wherein the firstagglomeration region is directly contacted to the first portion and thesecond agglomeration region is directly contacted to the second portionduring the sintering process, and wherein the thickness of the firstagglomeration region is less than that of the second agglomerationregion; removing the mandril after cooling; injecting a working fluidinto the hollow tube; and sealing the open end of the hollow tube.

In another embodiment of the disclosure, an instrument employed by theaforementioned method is provided. The instrument for fabricating a heatpipe is a mandril, wherein the mandril comprises at least one firstportion and at least one second portion, and wherein the first portionhas a thermal expansion coefficient larger than that of the secondportion.

A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 shows a flow chart of a method for fabricating a heat pipeaccording to an embodiment of the disclosure.

FIGS. 2-7 are a series of cross-section views showing a method forfabricating a heat pipe according to an embodiment of the disclosure.

FIG. 8 is a cross-section view of a heat pipe according to anotherembodiment of the disclosure.

FIG. 9 is a cross-section view of a heat pipe according to yet anotherembodiment of the disclosure.

DETAILED DESCRIPTION

The following description is of the best-contemplated mode of carryingout the disclosure. This description is made for the purpose ofillustrating the general principles of the disclosure and should not betaken in a limiting sense. The scope of the disclosure is bestdetermined by reference to the appended claims.

The disclosure provides a method for fabricating a heat pipe, and aninstrument employed by the method. A key aspect of the method forfabricating a heat pipe is to use a mandril having portions withdifferent thermal expansion coefficients. After performing a sinteringprocess, the powder filled into the hollow tube forms a sintered layerwith various thicknesses (i.e. the sintered layer exhibits various poresizes, porosity, and permeability simultaneously). Therefore, the heatpipe can have an evaporating section with low porosity (enhancing thecapillary force), and a condensing section and an adiabatic section withhigh porosity (improving the liquid transport), promoting the heattransfer performance of the heat pipe.

According to an embodiment of the disclosure, the method for fabricatinga heat pipe includes the following steps. A hollow tube is provided,wherein the hollow tube has an open end and a closed end. A mandril isdisposed into the hollow tube from the open end, wherein the inside wallof the hollow tube is separated from the mandril by a space, and whereinthe mandril comprises at least one first portion and at least one secondportion and the first portion has a thermal expansion coefficient largerthan that of the second portion. The space between the mandril and thehollow tube is filled with a powder. A sintering process is performed tothe hollow tube, forcing the powder to form at least one firstagglomeration region and at least one second agglomeration region,wherein the first agglomeration region is directly contacted to thefirst portion and the second agglomeration region is directly contactedto the second portion during the sintering process, and wherein thethickness of the first agglomeration region is less than that of thesecond agglomeration region. The mandril is removed after cooling. Aworking fluid is injected into the hollow tube. The open end of thehollow tube is sealed.

Since the mandril has portions with different thermal expansioncoefficients and the first portion with higher thermal expansioncoefficient has an expansion volume higher than that of the secondportion with lower thermal expansion coefficient during the sinteringprocess, the sintered layer has various thicknesses (and/or densities).Therefore, the sintered layer can be defined as a first agglomerationregion (lower thickness) and a second agglomeration region (higherthickness) for serving as an evaporating section, orcondensing/adiabatic sections.

Further, the disclosure also provides an instrument employing by theabove method. The instrument is a mandril, wherein the mandril comprisesat least one first portion and at least one second portion, and whereinthe first portion has a thermal expansion coefficient larger than thatof the second portion.

The following examples are intended to illustrate the invention morefully without limiting the scope, since numerous modifications andvariations will be apparent to those skilled in this art.

According to an embodiment of the disclosure, the method for fabricatinga heat pipe can include the following steps, as shown in FIG. 1:

First, a hollow tube 10 is provided (step 101), the hollow tube 10 has aclosed end 11 and an open end 13, as shown in FIG. 2. The hollow tube 10can be made of a metal or alloy with high thermal conductivity, such asCu, Al, Fe, Ni, Ti, alloy thereof, or stainless steel. The outsidecross-sectional shape of the hollow tube can be circular, ellipsoidal,polygonal, or combinations thereof, and the inside cross-sectional shapeof the hollow tube is circular, ellipsoidal, polygonal, or combinationsthereof.

Next, a mandril 12 can be disposed into the hollow tube 10 from the openend 13, and the mandril 12 is separated from the inside wall 15 of thehollow tube 10 by a space (step 102), as shown in FIG. 3. Particularly,the mandril 12 includes a first portion 12 a and a second portion 12 b,and the first portion 12 a has a thermal expansion coefficient largerthan that of the second portion 12 b.

Generally, the mandril can be made of Al, Al-containing alloy, orstainless steel. It should be noted that, in order to remove the mandril12 in the subsequent step, the mandril 12 of the disclosure has a singlecross-sectional area and a single cross-sectional shape at roomtemperature (such as 25° C.). The cross-sectional shape of the mandrilcan be circular, ellipsoidal, polygonal, or combinations thereof. A keyaspect of the method for fabricating a heat pipe of the disclosure is touse a mandril 12 having portions with different thermal expansioncoefficients. Due to the thermal expansion coefficient difference of thefirst portion 12 a and the second portion 12 b of the mandril 12, theexpansion volume of the first portion 12 a is larger than the expansionvolume of the second portion 12 b, resulting in a sintered layer withvarious thickness. Accordingly, a key aspect of the disclosure is tomodify the thermal expansion coefficient difference between the firstportion 12 a and the second portion 12 b, and will be discussed indetail below.

Next, a powder 16 is filled into the space 14 between the mandril 12 andthe hollow tube 10 (step 103), as shown in FIG. 4. Meanwhile, the powder16 disposed in the space 14 has a thickness T. The used powder is asingle type (i.e. the powder has a single particular size), such ascopper powder, titanium powder, nano carbon particle or combinationsthereof.

Next, the hollow tube 12 is subjected to a sintering process 50, therebyforcing the powder 16 to form a sintered layer 18 (step 104), as shownin FIG. 5. In the sintering process 50, the hollow tube 10 is sinteredunder a fixed sintering temperature (i.e. the whole hollow tube issintered under the same fixed sintering temperature), and the fixedsintering temperature can be of between 900-1100° C. It should be notedthat, since the first portion 12 a has a thermal expansion coefficientlarger than that of the second portion 12 b of the mandril 12, thepowder 16 adjacent to the first portion 12 a receives a higher extrusionpressure during the sintering process, forming a first agglomerationregion 18 a with a lower thickness T1 (having a compression ratio C1(C1=T1/T)). On the other hand, the powder 16 adjacent to the secondportion 12 b receives a lower extrusion pressure during the sinteringprocess, forming a second agglomeration region 18 b with a higherthickness T2 (having a compression ratio C2 (C2=T2/T))

Generally, the compression ratios C1 and C2 can be equal to or more than0.01, preferably of between 0.1-0.9. In this embodiment, the firstagglomeration region 18 a can serve as an evaporating section due to thehigh density, and low porosity (enhancing the capillary force). Further,the second agglomeration region 18 b can serve as an adiabatic sectionand a condensing section due to the low density, high porosity and highpermeability (improving the liquid transport).

In order to form a first agglomeration region 18 a with high density anda second agglomeration region 18 b with low density, the mandril 12(serving as the instrument of the method) should have a plurality ofportions with different thermal expansion coefficients. The mandril 12with various different thermal expansion coefficients can be obtainedvia a thermal treatment by controlling the metallographic structure andcomponents of the mandril. For example, the mandril has a portion madeof

304

stainless steel (serving as the first portion 12 a) and another portionmade of

430

stainless steel (serving as the second portion 12 b). Substantially, thethermal expansion coefficient of the stainless steel depends on themetallographic structure. For example,

304

stainless steel is classified as Austenitic stainless steel, and

430

stainless steel is classified as Ferritic stainless steel. Austeniticstainless steel generally has a thermal expansion coefficient largerthan that of Ferritic stainless steel. The

304

stainless steel and

430

stainless steel are the most widely used in industrial applications, andthe

304

stainless steel has a thermal expansion coefficient approximately 1.5times larger than that of the

430

stainless steel (

304

stainless steel consists essentially of chromium from 18.0% to 20.0%,nickel from 8.0% to 11.0%, and carbon 0.08% in maximum, and have athermal expansion coefficient of 18.8×10E-6 m/m-K at 982° C.;

430

stainless steel consists essentially of chromium from 14.0% to 18.0%,and carbon 0.12% in maximum, and have a thermal expansion coefficient of11.9×10E-6 m/m-K at 982° C.).

The ratio between the thermal expansion coefficient of the first portionof the mandril and the thermal expansion coefficient of the secondportion of the mandril can be between 1.01 and 2.00.

The selection of the thermal expansion coefficient ratio between thefirst and second portions depends on the thickness of the powder 16 andthe applications of the obtained heat pipe. For example, when thethickness of the powder is less than 0.3 mm, the thermal expansioncoefficient ratio between the first and second portions can be between1.01 and 1.1. Further, when the thickness of the powder is less than 10mm, the thermal expansion coefficient ratio between the first and secondportions should be less than 2.00 to prevent a breakpoint from beingformed at the interface between the first agglomeration region 18 a andthe second agglomeration region 18 b. Table 1 lists the suitablematerials for serving as first or second portions of the mandril of thedisclosure (the first and second portions are made of stainless steelwith different thermal expansion coefficients):

TABLE 1 liner thermal expansion coefficient stainless steel (μm/m-K)Austenitic stainless steel 304 18.8 Austenitic stainless steel 310 14.4Austenitic stainless steel 316 16 Ferritic stainless steel 410 9.9Ferritic stainless steel 430 11.9

After the sintering process, the disclosure further includes the belowsteps:

As shown in FIG. 6, after the sintering process 50, the hollow tube iscooled down to room temperature, and then the mandril is removed (step105). Next, a working fluid 20 is injected into the hollow tube (step106) from the open end, and then the open end of the hollow tube issealed (step 107), obtaining a heat pipe 100, as shown in FIG. 7. Theworking fluid can be water, ammonium hydroxide, methanol, acetone,heptane, or combinations thereof.

The heat pipe obtained by the aforementioned method includes a firstagglomeration region 18 a serving as an evaporating section due to thehigh density, and low porosity, and a second agglomeration region 18 bserving as an adiabatic section and a condensing section due to the lowdensity, high porosity and high permeability.

In general, since the evaporating section of the heat pipe 100 is usedto be in contact with a heat source of an electric device, the firstagglomeration region 18 a can be optionally located on the centralposition of the heat pipe 100, except for being located on one side ofthe heat pipe 100, as shown in FIG. 8. Moreover, if the electric devicehas a plurality of heat sources, the heat pipe 100 could have aplurality of the first agglomeration region 18 a serving as theevaporating section (corresponding to the heat source), as shown in FIG.9.

Accordingly, the mandril of the disclosure having portions withdifferent thermal expansion coefficient can be obtained via a thermaltreatment to control the metallographic structure. Therefore, the methodfor fabricating a heat pipe employed by the aforementioned mandril canproduce a heat pipe having a sintered layer with various porosities,improving the heat transfer performance. Further, since the powder usedin the disclosure can have a fixed particular size, the product yieldcan be increased and the process complexity can be reduced.

While the disclosure has been described by way of example and in termsof the preferred embodiments, it is to be understood that the disclosureis not limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

1. A method for fabricating a heat pipe, comprising: providing a hollowtube, wherein the hollow tube has an open end and a closed end;disposing a mandril into the hollow tube from the open end, wherein theinside wall of the hollow tube is separated from the mandril by a space,and wherein the mandril comprises at least one first portion and atleast one second portion and the first portion has a thermal expansioncoefficient larger than that of the second portion; filling up the spacebetween the mandril and the hollow tube with a powder; performing asintering process to the hollow tube, forcing the powder to form atleast one first agglomeration region and at least one secondagglomeration region, wherein the first agglomeration region is directlycontacted to the first portion and the second agglomeration region isdirectly contacted to the second portion during the sintering process,and wherein the thickness of the first agglomeration region is less thanthat of the second agglomeration region; removing the mandril aftercooling; injecting a working fluid into the hollow tube; and sealing theopen end of the hollow tube.
 2. The method as claimed in claim 1,wherein the mandril is made of a stainless steel, and wherein the ratiobetween the thermal expansion coefficient of the first portion and thethermal expansion coefficient of the second portion is of between1.01-2.00.
 3. The method as claimed in claim 1, wherein the hollow tubeis made of Cu, Al, Fe, Ni, Ti, or alloy thereof.
 4. The method asclaimed in claim 1, wherein the powder comprises copper powder, titaniumpowder, nano carbon particle, or combinations thereof.
 5. The method asclaimed in claim 1, wherein the working fluid comprises water, ammoniumhydroxide, methanol, acetone, heptane, or combinations thereof.
 6. Themethod as claimed in claim 1, wherein the mandril has a singlecross-sectional area and a single cross-sectional shape.
 7. The methodas claimed in claim 1, wherein an outside cross-sectional shape of thehollow tube is circular, ellipsoidal, polygonal, or combinationsthereof.
 8. The method as claimed in claim 1, wherein an insidecross-sectional shape of the hollow tube is circular, ellipsoidal,polygonal, or combinations thereof.
 9. The method as claimed in claim 6,wherein the single cross-sectional shape of the mandril is circular,ellipsoidal, polygonal, or combinations thereof.
 10. An instrument forfabricating a heat pipe, which is a mandril, wherein the mandrilcomprises at least one first portion and at least one second portion,and wherein the first portion has a thermal expansion coefficient largerthan that of the second portion.
 11. The instrument as claimed in claim10, wherein the mandril is made of a stainless steel, and wherein theratio between the thermal expansion coefficient of the first portion andthe thermal expansion coefficient of the second portion is of between1.01-2.00.
 12. The instrument as claimed in claim 10, wherein themandril has a single cross-sectional area and a single cross-sectionalshape before subjecting to a thermal treatment.
 13. The instrument asclaimed in claim 12, wherein the cross-sectional shape of the mandril iscircular, ellipsoidal, polygonal, or combinations thereof.